I use Due – a simple reminder app – virtually every day on both my iPhone and Mac. There’s a lot of reminder options on iOS, but I haven’t found anything that comes close to the fluidity and speed of Due’s UI. The syncing also works slick between iPhone, iPad and Mac clients (via Dropbox or iCloud, your choice.)
The app rarely goes on sale, but they dropped the price this week from $5 to $3. If you’re in the market it’s well worth your time.